Patent applications No. 20,910 A/86 and 19,494 A/87 in the name of the Applicant hereof disclose microemulsions comprising perfluoropolyethers (PFPE) having perfluoroalkyl end groups, obtained by using fluorinated surfactants and optionally fluorinated or non-fluorinated co-surfactants, such as alkanols and fluoroalkanols, and optionally electrolytes.
Generally, in the abovesaid microemulsions the maximum amount of solubilized PFPE in the aqueous phase was depending on the utilized amounts of surfactant and of co-surfactant, as the maximum obtainable PFPE/water interphase could be considered as lower than, or at the most equal to the sum of the areas of the surfactant and co-surfactant polar heads.
Therefore it would be very useful, for example for uses in the polymerization of fluorinated monomers, as is described in patent application Ser. No. 20,909 A/86 in the name of the Applicant, to have available a microemulsion characterized by the presence of a water-soluble surfactant and co-surfactant in an amount lower than the one which is required for the microemulsions of PFPE having perfluoroalkyl end groups.
It has now surprisingly been found that it is possible to prepare microemulsions characterized by a lower concentration of surfactant and co-surfactant and even free from added water-soluble perfluorinated surfactant, if a PFPE having end groups at least partially of the non-perfluoroalkyl type is used.
Thus, an object of the present invention are microemulsions of PFPE having at least partially non-perfluoroalkyl end groups. By "microemulsion" there are usually meant products macroscopically consisting of a single liquid, transparent or opalescent and optically isotropic phase, comprising two immiscible liquids and at least a surfactant, in which products one of the two immiscible liquids is dispersed in the other in the from of droplets having diameters ranging from about 50 to 2,000 A.degree..
In principle it cannot be excluded that particles having higher or lower sizes up to the limit of the molecular dispersion may be present. Furthermore, structures may be present, in which both liquids, interdispersed as bicontinuous tri-dimensional immiscible films are co-solubilized at a molecular level.
Such products spontaneously form by simple mixing of the components and are indefinitely stable in a certain temperature range, defined later on as existence range.
Whenever used herein, the term "microemulsion" has a broader meaning, comprising also non optically isotropic systems (i.e. birefractive), characterized by an orientation of the liquid-crystalline type of the components.
The perfluoropolyethers of the present invention are characterized in that they can be a mixture of components of different molecular weight, comprising a perfluoropolyether chain, in which a few end groups do not consist of perfluoroalkyl groups but of functional groups.
Such end groups may be monovalent or polyvalent.
Furthermore, the functional groups can be present on ramifications of the perfluoropolyethereal chain, as is described in Italian patent application No. 20,346 A/86.
The preferred average functionality of the molecules is in the range from 0.1 to 4, preferably from 0.1 to 2 and most preferably from 0.3 to 1.
The functional groups present in the PFPE chains are representable by the formula: EQU --(B).sub.n --T.sub.m
wherein n=0 or 1, B is a divalent or polyvalent, in particular an alkylene or a cycloalkylene or an arylene linking hydrocarbon radical, having up to 20 carbon atoms and preferably up to 8 carbon atoms, m varies from 1 to 3 and preferably is 1, and T is one of the following groups or radicals: --H, --COOH, --SO.sub.3 H, --OH, --polyoxyalkylene--OH, an ester or amidic group, or an aminic or quaternary ammonium group. The preferred end groups are: --COOH, --OH, --polyoxyalkylene--OH, quaternary ammonium groups. The acid end group is preferably salified.
As starting products it is possible to use also perfluoropolyethers with --COF or --SO.sub.2 F end groups which, in the preparation of the microemulsion, hydrolize to --COOH and --SO.sub.3 H groups.
The average molecular weight of the perfluoropolyethereal chain ranges from 1,500 to 10,000, and preferably from 2,000 to 6,000.
Generally, aqueous microemulsions comprising perfluoropolyethers having end groups with hydrophilic functionality (for example COOH, polyoxyalkylene--OH, etc.) exhibit the property of requiring, for being formed, a lower surfactant amount with respect to corresponding microemulsions of PFPE having perfluoroalkyl end groups, because the hydrophilic groups co-operate in the formation of an interphase film, which renders the perfluorooxyalkylene chains compatible with the aqueous solution, although the functionalized PFPE molecules are substantially insoluble. At the limit, it is possible to prepare functional PFPE microemulsions with hydrophilic functionality even in the total absence of added surfactant and only in the presence of a co-surfactant, which should be preferably hydrogenated.
Preferred perfluoropolyethers for the preparation of the microemulsions according to the invention, having R.sub.f, R'.sub.f end groups of the functionalized type and optionally in part also of the perfluoroalkyl type, as pointed out above, are the ones consisting of fluorooxyalkylene units selected from the following: ##STR1## and in particular belonging to the following classes: ##STR2## with a random distribution of the perfluorooxyalkylene units, where m, n, p have such average values as to meet the abovesaid requirements regarding the previously indicated molecular weight and R.sub.f and R'.sub.f are functional groups, and optionally in part perfluoroalkyl groups;
2) R.sub.f O(CF.sub.2 CF.sub.2 O).sub.n (CF.sub.2 O).sub.m R'.sub.f
with a random distribution of the perfluorooxyalkylene units, wherein m, n have such values as to meet the abovesaid requirements; ##STR3## wherein m, n, p, o have such values as to meet the above-indicated requirements; ##STR4## wherein n has such a value as to meet the above-indicated requirements;
5) R.sub.f O(CF.sub.2 CF.sub.2 O).sub.n R'.sub.f
wherein n has such an average value as to meet the abovesaid requirements;
6) R.sub.f O(CF.sub.2 CF.sub.2 CF.sub.2 O).sub.n R'.sub.f or R.sub.f O(CH.sub.2 CF.sub.2 CF.sub.2 O).sub.n R'.sub.f
wherein n has such a value as to meet the abovesaid requirements.
Perfluoropolyethers of class (1) are known on the market under the trade-mark Fomblin.RTM. Y, those of class (2) under the trade-mark Fomblin.RTM. Z, all of them being manufactured by Montedison. Commercially known products of class (4) are Krytox.RTM. (DuPont).
The products of class (5) are described in U.S. Pat. No. 4,523,039; those of class (6) are described in European patent No. EP 148,482 to Daikin.
The ones of class (3) are prepared according to U.S. Pat. No. 3,665,041. Suitable are also the perfluoropolyethers described in U.S. Pat. No. 4,523,039 or in J. Am. Chem. Soc. 107, 1195-1201 (1985).
The products of classes 1, 2, 3, which are obtained through photochemical oxidation processes, are utilizable as raw products of the photo-oxidation process, which contain peroxide groups --OO-- in the chain, said groups being inserted between the perfluorooxyalkylene units.
Perfluoropolyethers suited to the preparation of the microemulsions according to the invention are also the ones disclosed in Italian patent application No. 20,346 A/86, which comprise functional groups of the above-indicated type arranged along the chain and functional or perfluoroalkyl end groups.
The fluorinated surfactants to be used according to the present invention may be ionic or non-ionic. In particular, the following ones can be cited:
(a) perfluorocarboxylic acids with 5 to 11 carbon atoms and the salts thereof; PA1 (b) perfluorosulphonic acids with 5 to 11 carbon atoms and the salts thereof; PA1 (c) the non-ionic surfactants cited in European patent application No. 0051526 consisting of a perfluoroalkyl chain and of a polyoxyalkylene hydrophilic head; PA1 (d) mono- and bi-carboxylic acids derived from perfluoropolyethers having mean molecular weight not higher than 1000, and the salts thereof; PA1 (e) the non-ionic surfactants consisting of a perfluoropolyethereal chain bound to a polyoxyalkylene chain; PA1 (f) perfluorinated cationic surfactants or those derived from perfluoropolyethers having 1, 2 or 3 hydrophobic chains.
As a co-surfactant it is possible to use a hydrogenated alcohol having 1 to 12 carbon atoms, preferably 1 to 6 carbon atoms.